This invention relates to a switched-mode power supply circuit having an operating state and a standby state and comprising a controllable first switch arranged in series with the primary winding of a transformer. The series arrangement thus formed is coupled to the terminals of a d.c. input voltage. The transformer has a first secondary winding for providing a first d.c. output voltage in the operating state and a second secondary winding for providing a second d.c. output voltage. The circuit also comprises a second switch for bringing the supply circuit into the stand-by state in which the value of the first output voltage is considerably lower than in the operating state, and a third switch controllable by means of the second switch and coupled to the second secondary winding for maintaining the second output voltage in the stand-by state at substantially the same value as in the operating state. The circuit further comprises a comparison stage for comparing an output voltage with a reference voltage and for generating a control signal for controlling the duration of periodically occurring drive pulses applied to the first switch.
A power supply circuit of this type is known from German Patent Application No. 3,223,756. In this known circuit, which is intended for use in a television receiver, both the first output voltage and other output voltages, which are derived from secondary windings of the transformer, have a lower value in the stand-by state than in the operating state so that the power consumption is smaller, while the second output voltage, to which a remote control is connected, has substantially the same value. In the stand-by state the first switch continues to operate normally, though every time with a shorter conduction period than in the operating state. This continuous operation at a high frequency, i.e. 25 to 30 kHz involves, however, quite a considerable power dissipation.